1. Field of the Invention
The present invention relates to a water-repellent sheet having a water-repellent surface protected by a protective film easily removable when used. The water-repellent sheet of the present invention can be suitably used as a sheet for preventing snow adhesion outdoors or for preventing water adhesion, utilizing water repellency.
2. Description of the Related Art
To prevent damage from snow or water to an outdoor surface such as a billboard, sign, car body or building exterior, it has been proposed to form a water-repellent layer on such a board to protect it. A water-repellent layer of this kind often contains a fluorocarbon-based polymer.
For example, Japanese Patent Laid-Open Publication No. 61-23656 discloses a composition that contains a perfluoro-compound having an extremely small intermolecular cohesive force and having good chemical stability and low surface tension. The composition contains a specific amount of a polymer of (metha)acrylate monomer having a perfluoro-alkyl group with a specific number of carbon atoms, and reduces water adhesion and snow adhesion. In addition, Japanese Patent Laid-Open No. 10-88061 discloses a composition for preventing water adhesion and snow adhesion containing a fluorocarbon-based polymer binder and a fluorocarbon-based resin powder dispersed in the binder. Application of these compositions on the surfaces of structures can form water-repellent layers on the surfaces.
When water-repellent compositions as discussed above are used, the liquid composition is often applied directly to a board at a construction site and the liquid composition is dried to form a water-repellent layer. The application of the liquid composition requires a relatively long period of time and substantial labor, and thus is inconvenient. In addition, the properties of the surface coating (water-repellent layer) after drying can depend on the weather at the construction site during application, and so water-repellent layers with consistent properties are difficult to form.
There is proposed a water-repellent sheet for a board comprising a base layer having first and second major surfaces, a water-repellent layer having a water- and snow-repellent composition applied to the first surface of the base layer, and an adhesive layer applied to the water-repellent second major surface. When such a water-repellent sheet is prepared in advance and is bonded to a board at a construction site, a water-repellent board having a water-repellent layer on the outermost surface thereof can be readily produced. Such a water-repellent sheet does not require the application of a liquid composition at the construction site, makes construction of a water-repellent board extremely simple and makes formation of a water-repellent layer independent of weather conditions at the construction site, thereby allowing for construction of water-repellent boards having consistent properties.
National Publication of International Patent Application No. 2001-511477 (corresponding to International Publication WO099/05345) discloses an example of such a water-repellent sheet in which a fluorocarbon-based polymer or a fluorocarbon-based polymer composition is applied on a surface in the form of an organic solution or a water-based organic dispersion. This sheet, obtained by selecting fluorocarbon-based polymers having a particular type of chemical structure, characteristically exhibits good thermal stability even when melt processed at 300° C. or higher (this temperature being a common process condition).
In addition, Japanese Patent Laid-Open No. 2-145673 discloses a water-repellent sheet produced by forming an adhesive layer, a base layer and a water-repellent layer in this order on a release sheet (i.e., liner). This sheet is applied to a specific location on a board whereby the release sheet is removed during use.
Furthermore, Japanese Patent Laid-Open No. 5-92350 discloses a fluorocarbon resin laminate made by modifying the surface of a sheet or film of fluorocarbon resin by electric discharge treatment of the surface to improve the adhesion of an adhesive layer to the surface where the adhesive layer is then used to adhere the laminate to a board. The electric discharge treatment is performed in an inert gas atmosphere containing an organic compound having a functional group. The adhesive is a polymer obtained by polymerizing or copolymerizing a monomer having the same functional group as that of the organic compound or having a functional group with affinity for that of the organic compound.
Still further, National Publication of International Patent Application No. 2001-520127 (corresponding to International Publication WO99/19414) discloses a protective film having an outer surface layer of a fluorocarbon polymer. This protective film is produced by forming a layer of fluorocarbon polymer on a first release liner and hardening the layer somewhat; applying an adhesive layer to a second release liner and sufficiently hardening the release layer; and subsequently applying an inter-penetrating polymer layer between the adhesive layer and the fluorocarbon polymer surface layer to heat cure it. The second release liner can be released prior to applying the adhesive layer to a board; the first release liner is released from the surface of the fluorocarbon polymer later, but before its use as a protective film. The face of the release liner which is in close contact with the fluorocarbon polymer surface layer is normally treated with a release agent. Accordingly, the release liner is not strongly bonded to the surface of the fluorocarbon polymer layer and thus can be easily removed from the fluorocarbon polymer surface layer.
However, water-repellent sheets can exhibit reduced water-repellency when the surface of the water-repellent layer has been damaged by friction with another material before it is applied to a board at a construction site. For example, bonding a water-repellent sheet to a board can be done by rubbing the surface of the water-repellent layer of the water-repellent sheet with a compression bonding jig such as a squeegee while using pressure to prevent appearance defects due to bubbles being entrapped between the water-repellent sheet and the board. However, during this type of application, the surface of the water-repellent layer can be damaged and this can reduce the water repellency due to exposure of the base layer surface, etc.
Furthermore, surface protective films are used for the protection of coatings such as the surface coatings of auto bodies. Surface protective films that can be used include self-adhesive films and adhesive films made by applying an adhesive layer to a polymer film such as polyester (PET).
Surface protective films must be removable from the surface to be protected. In addition, as discussed above, when a water-repellent sheet is bonded to a board, it is compression-bonded by pressing a compression-bonding jig to the water-repellent sheet surface. When the surface of a water-repellent layer is protected with a protective film, the protective film may deadhere from the water-repellent layer during the compression bonding operation if the adhesion between the protective film and the water-repellent layer is insufficient. Deadherence of the protective film during such a compression-bonding operation interferes with the compression bonding operation. Therefore, the protective film should not easily deadhere from the surface of the water-repellent layer. However, none of the patents described above discloses specific means for effectively protecting a water-repellent layer using a protective film.